The invention relates to a delay estimation method used in a radio system comprising as a transmitter and a receiver at least one base station and a subscriber terminal, in which system signals comprise waveforms and wherein a received or preprocessed signal is sampled and wherein the mutual delays are estimated from the received signals, which originate from one or several transmitters and which have propagated typically along several paths.
The invention also relates to a receiver arranged to be used in a radio system comprising as transmitters and receivers at least one base station and a subscriber terminal having signals that contain waveforms, in which system a receiver is arranged to sample a received or preprocessed signal and to estimate the mutual delays from the signals, which originate from one or several transmitters and which are typically muitipath signals that have propagated along several paths.
In a radio system, after a signal has left the transmitter it is scattered into several parts as it propagates and therefore several copies of the same signal arrive at the receiver at slightly different times, since the copies have propagated at the same speed along different paths. The signal scattering varies depending on the environment, for example mountains, countryside, suburbs, city centre etc. The greater the scattering caused by the environment, the more paths are required for modelling one signal.
One of the central problems in designing and implementing data transmission systems is the simultaneous transmission and reception of signals of several simultaneous users so that the signals interfere with one another as little as possible. For this reason and due to the transmission capacity used, several different transmission protocols and multiple access methods, such as the code division multiple access (CDMA) method, have been developed.
CODMA is a multiple access method that is based on the spread spectrum technique and that has been recently applied in cellular radio systems in addition to the prior FMDA (frequency division multiple access) and TDMA (time division multiple access). CDMA has several advantages compared to the prior methods, such as simplicity of the frequency planning and spectral efficiency. In the following, the invention will be described when applied by way of example in a direct spreading CDMA system. The application of the invention to other systems, such as a digital car radio, is also possible.
In the direct spreading CDMA method, a narrow-band data signal of a user is multiplied to a relatively broad band with a spreading code having a considerably broader band than the data signal. Bandwidths used in known test systems include 1.25 MHz, 10 MHz and 25 MHz. In connection with multiplying, the data signal spreads to the entire band to be used. All users transmit simultaneously by using the same frequency band. A separate spreading code is used over each connection between the base station and a mobile station, and the user signals can be distinguished from one another at the receivers on the basis of the spreading code of each user. The spreading codes are preferably selected in such a way that they are mutually substantially orthogonal, i.e. they correlate with one another as little as possible.
Correlators provided in CDMA receivers implemented in a conventional manner are synchronized with a desired signal that is identified on the basis of the spreading code. The data signal is restored in the receiver to the original band by multiplying it again with the same spreading code that was used during the transmission. The signals that have been multiplied with some other spreading code do not ideally correlate and are thus not restored to the narrow band. Therefore they appear as noise for the detected signal. The scattering of the signals into a multipath signal and the signals of the other users complicate the detection of the desired signal by distorting the received signal.
The detection of the user symbols is possible when the connection between the transmitter and the receiver is synchronised, i.e. the signal delay is known. A prior art method is to use a conventional one-user delay estimator. This method works well only when certain boundary conditions apply, and since the implementation is simple its capacity is also insufficient. Successful synchronization typically requires advance information on the user symbols and the complex attenuation coefficients of the channel. Especially in asynchronous systems, i.e. systems where the signals of the users are not mutually synchronised, reliable signal reception is difficult since the user symbols are disturbed by several symbols of the other users. Filters adapted to spreading codes and sliding correlators used as detectors in conventional receivers are not effective, however. More effective known methods include multiuser detectors, e.g. a decorrelating detector that eliminates multiple access interference from the received signal by multiplying it with the cross-correlation matrix of the spreading codes used. A decorrelating detector and the known implementations thereof are described in greater detail in Linear multiuser detectors for synchronous code-division multiple access channels by Verdu Lupas, IEEE Transactions on Information Theory, vol 35, no. 1, pp 123-136, January 1989, and Near-far resistance of multiuser detectors in asynchronous channels, by Verdu Lupas, IEEE Transactions on Communications, vol 38, April 1990, which are incorporated herein by reference.
The purpose of the present invention is to provide a delay estimation method wherein delay estimates are formed simultaneously for all users and paths before the detection of user symbols and without information about the properties of the channel.
This is achieved with a method of the type disclosed in the preamble, characterized by determining the paths to be estimated, determining the code bank model to be used which contains information on the waveforms used, selecting one of the determined paths at a time for estimation, and with respect to these paths, solving the least squares minimization problem between the received signal and the code bank model such that the delay of only one selected path is changed at a time within the desired search interval while the delays of the other paths remain unchanged, so that delay-specific minimization results are formed, searching for a delay which corresponds to the minimization results and for which the minimized result is smaller than or equal to the results for the other delays of the same path, storing said delay which is the delay estimate that was searched for, and utilizing said delay as a constant value of said path when the delays of the other paths are searched for.
The receiver according to the invention is characterized in that the receiver comprises path model means, code bank model means and delay means, said delay means are arranged to receive a signal, information on the paths to be estimated from the path model means, and the code bank model from the code bank model means, said delay means are also arranged to carry out the following delay estimation procedures specifically for each path: solving the least squares minimization problem between the received signal and the code bank model of the code bank model means in order to provide minimization results, in which minimization problem the delays of only one path at a time are to be changed while the delays of the other paths remain unchanged, searching for a delay for which the minimization result is smaller than or equal to the results for the other delays of the same path, and storing said delay which is the delay estimate that was searched for, and using said delay as a constant value when the delays of the other paths are searched for.
The method according to the invention and the receiver applying the method provide considerable advantages. The delays of all the paths of all the users can be taken into account simultaneously. It is not necessary to know the user symbols and the complex attenuation coefficients of the channel when the delay estimates are formed. Also, known parameters can be taken into account and combined with the method.